In an electromagnetic feeder used to convey materials or an electromagnetic screen used to sieve out materials, a trough or a sieving net is vibrated by AC excitation of an electromagnet.
More specifically, in these electromagnetic vibrating apparatuses, two masses are connected by a resilient member, and the resonant frequency of the structure is set to a level close to a power source frequency to enlarge a small electromagnetic force to a large one thereby applying strong vibrations to the trough or the sieving net.
A conventional electromagnetic feeder is excited with the number of vibrations having an audio frequency of a commercial power source, 50 Hz or 60 Hz. Therefore, in the case of the large type feeder, vibrational noises are violent and unpleasant, producing an environmental problem. Furthermore, as to the vibrating force, in the conventional electromagnetic feeder, only half the generated attractive force is utilized, providing the electromagnetic feeder with an extremely low rate of efficiency. In the case of the small feeder, this poses no significant problem but, in case of the large feeder, with a large current flow, there is a correspondingly large energy loss.